Self-Sealing Electrical Cable Using Rubber Resins

ABSTRACT

An electrical cable and a method for manufacturing the electrical cable are provided in which a plurality of insulated conductors have an inner protective layer extruded thereabout. A plurality of longitudinally extending ribs or fins or exterior ribbed or finned surfaces are formed outward of the inner protective layer between which exist a plurality of voids. An outer insulation layer can be formed in the same operation as the fins or ribbed surface and the inner layer or in a subsequent operation. A self-sealing elastomeric material is applied to the conductor surface or is present between the fins and between the inner protective layer and the outer insulation layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending applicationSer. No. 10/364,808, filed Feb. 11, 2003, which is acontinuation-in-part of application Ser. No. 09/851,475 filed May 8,2001 which is a continuation-in-part of co-pending application Ser. No.09/730,661 filed Dec. 6, 2000 which is a continuation-in-part of patentapplication Ser. No. 09/756,533 filed Jan. 8, 2001, which is a divisionof patent application Ser. No. 09/223,482 filed Jan. 11, 1999, now U.S.Pat. No. 6,184,473 all of which are relied on and incorporated byreference.

BACKGROUND OF THE INVENTION

Insulated solid and stranded electrical cables are well known in theart. Generally stranded cables include a central stranded conductor witha protecting insulation jacket disposed around the conductor.

The most frequent cause of failure of directly buried aluminum secondarycables is a cut or puncture in the insulation inflicted during or afterinstallation. This leads to alternating current corrosion of thealuminum and finally to an open circuit. When a conductor is exposed towet soil, upon damage, leakage current may flow, and cause localizedelectrochemical conversion of aluminum to hydrated aluminum oxide andeventually to an open circuit of the conductor.

In the U.S., thousands of such instances occur annually and the repair(location, excavation, repair, and replacement) can be very costly. As aresult of the failures and in response to this problem, a tougherinsulation system was introduced and became an industry standard. Thetougher cable is described as “ruggedized,” and generally consists oftwo layers: an inner layer of low density weight polyethylene and anouter layer of high density polyethylene. This design is more resistantto mechanical damage than one pass low density polyethylene, but stillcan result in exposure of the aluminum conductor if sufficient impact isinvolved.

Investigations show that AC electrolysis current can approach half-waverectification when the current density is high. This accounts for therapid loss of aluminum metal frequently experienced in the field. Acaustic solution (pH 10-12) develops at the aluminum surface anddissolves the protective oxide film.

The mechanism of aluminum cable failure is the formation of hydrousaluminum oxide. As the aluminum oxide solids build up, the insulation inthe vicinity of the puncture is forced to swell and splits open, makinglarger areas of the aluminum conductor surface available forelectrolysis, thus increasing the leakage current and accelerating thecorrosion process. Rapid loss of aluminum by AC electrolysis continuesuntil ultimately the cable is open-circuited. A caustic environment iscreated at the aluminum, electrolyte interface, which dissolves theprotective oxide film.

The ruggedized or abuse resistant type insulation was supposed toprotect the cable from physical abuse. While it helped this problem, itdid not eliminate 600 V cable failures. Utilities have recently reportedvarying numbers of 600 V aluminum underground distribution cable failurerates scattered between 70 and 7000 per year. Failures are evidenced byan open circuit condition accompanied by severe corrosion of thealuminum conductor.

All the reasons for 600 V failures are not known, but several have beenpostulated by cable users. These cables seem to experience a high degreeof infant mortality, followed by failures occurring over decades. Theinfant moralities are usually directly related to damage caused byadjacent utilities, damage inflicted by landscaping and planting, ordamage to the cable prior to or during installation. The failuresoccurring years later are harder to explain. There have beenpostulations of lightning damage, manufacturing defects, or insulationdegradation over the life of the installation.

In order to better understand the insulation characteristics, studies ofthe AC breakdown, and DC impulse breakdown were conducted. AC breakdownstudies on several different cables showed a high safety margin ofperformance. Each of these cables had a 0.080 inch wall thickness. Testswere conducted in water filled conduits. The AC breakdown strength ofall of these cables was consistently above 20 kV, far in excess of theoperating stress.

Impulse breakdown studies have also been performed on several 600 Vcable constructions having different insulation formulations. Theimpulse breakdown level of these cables was approximately 150 kV. Thisexceeds the BIL requirements of a 15 kV cable system and should wellexceed the impulses on 600 V secondary cables during operation.

The above margins of electrical performance were measured on new cables.They are far above what is needed to operate on a 600 V system sincemost of these cables operate at 120 V to ground. One of the tests duringcompound and product development is a long term insulation resistancetest performed in water at the rated operating temperature of theinsulation. For crosslinked polyethylene cables the water temperature is90° C. The insulation resistance must demonstrate stability and be aboveminimum values for a minimum of twelve weeks. If there is instabilityindicated, the test is continued indefinitely. Relative permittivity ismeasured at 80 v/mil and must meet specific values. Increase incapacitance and dissipation factor are also measured in 90° C. waterover a 14 day period. Insulation compounds used in present day cableseasily meet these requirements.

Manufacturing defects in cable insulation are found during production byeither of two methods. During the extrusion process, the cable is sentthrough a spark tester, where 28 kV DC, or 17 kV AC, is applied to theinsulation surface. Any manufacturing defect resulting in a hole in theinsulation will initiate a discharge, which is detected by the sparktester. Most manufacturers use this method. Another test that is alsooften employed is a full reel water immersion test. In this test 21 kVDC, or 7 kV AC is applied to the cable after immersion for 1 hour or 6hours, depending on whether the cable is a plexed assembly or singleconductor, respectively. The actual voltages used for these tests aredependent on the wall thickness. The above values are for an 0.080 inchwall.

The above testing has demonstrated electrical performance that is stableand far surpasses the requirements of the installation for 600 V cable.This does not explain a sudden cable failure after many years ofoperation. Such sudden failure can be explained by a betterunderstanding of the failure mechanism. Aluminum corrosion in thepresence of an alternating leakage current is a combination of twodifferent mechanisms. Aluminum is normally afforded a great deal ofcorrosion protection by a relatively thin barrier layer of aluminumoxide, and a more permeable bulk layer of oxide. However, flaws orcracks exist in these layers which provides a spot for the corrosionreaction to begin. The metal in contact with water undergoes an anodic(positive ions moving into solution) and a cathodic cycle, sixty timesper second.

During the anodic half cycle of leakage current, aluminum ions leave themetallic surface through these flaws and combine with hydroxyl ions inthe water surrounding the cable. This reaction results in pitting of themetal and the formation of aluminum hydroxide, the whitish powderevident in corroded cables. Another important reaction also occurs. Thehydroxyl ions are attracted to the metal surface during this half cycle,which increases the pH, causing a caustic deterioration of the oxidelayer, further exposing more aluminum.

During the cathodic half cycle another reaction occurs. Hydrogen ionsare driven to the aluminum surface. Instead of neutralizing the caustichydroxyl concentration, the hydrogen ions combine and form hydrogen gas,which leaves the cable. The hydrogen depletion has the effect of furtherconcentrating the caustic hydroxyl ions, thus furthering thedeterioration of the surface oxide. No pitting occurs during this halfcycle since the aluminum ion is attracted to the metal. As can be seen,a caustic solution develops, hydrogen evolves, aluminum pitting takesplace, and aluminum hydroxide forms during this full cycle reaction.

A critical current density is necessary to sustain the corrosionreaction. Below this current density corrosion will be very slight, oralmost imperceptible. Once the current density is high enough, thereaction can be swift. The necessary current density is below 1 mA/in².The current density of a damaged 600 V cable is influenced by thevoltage, leakage resistance, and the area of exposed metal. Variablesaffecting this can include dampness of the soil, chemistry of the soil,degree of damage, etc.

DESCRIPTION OF THE RELATED ART

The toughest cables on the market today will not always stand up to therigors of handling, installation, and operation. And exposed aluminumwill eventually deteriorate. The solution, then, is to find a way toeconomically prevent the corrosion process.

Attempts have been made to prevent the ingress of moisture byintroducing a sealant between the strands of the conductor and betweenthe conductor and the insulation. See U.S. Pat. Nos. 3,943,271 and4,130,450. However, it has been found that the mere introduction of asealant into such spaces is not entirely satisfactory. Attempts toprevent moisture from reaching the conductor, such as using waterswellable material, have not met with technical and/or economic success.For example, voids may be formed in the sealant during the applicationthereof or may be formed if the cable is accidentally punctured. Anysuch spaces or voids form locations for the ingress of moisture whichcan lead to corrosion of the conductor and conventional sealants used inthe cables cannot eliminate such voids.

A prior art attempt to minimize the flow of moisture or water within theinterstitial spaces of a stranded conductor came in the form ofcompacted or compressed stranded conductors. The stranded conductoritself was radially crushed in order to reduce the diameter of theconductor and to fill the interstitial spacing with metal from theindividual wires themselves. The drawback to this method is that eventhough some deformation of the individual wires does take place, andsome of the interstitial spacing is filled, there is still thepossibility of cable insulation damage through which moisture can enterthe cable and contact the conductor.

Another attempt at correcting moisture flowing within interstitial spaceconsisted of filling the interstitial space with a foreign substancewhich physically prevented the flow of the moisture or water within theconductor structure. These substances typically comprised some type ofjelly base and a polyethylene filler material. At slightly elevatedtemperatures, this compound becomes fluid and viscous and can be appliedas the conductor is being formed. The individual wires used to form theconductor are fed into an extrusion die where the moisture blockingcompound is extruded onto and around each individual wire and, as thewires are stranded into the conductor, the interstitial space is filledwith the jelly-like material. Upon cooling, the filler becomes verystable and immobile and does not flow out of the interstitial spaces ofthe stranded conductor. Once the filling compound is applied within theinterstitial spaces of the stranded conductor, it tends to remain inplace. The problems encountered in applying such a filling substancerevolve around precise metering of the material into the interstitialspaces as the stranded conductor is being formed. If too much materialis extruded into the conductor, the outer insulation will not fitproperly. If too little material is applied, the interstitial spaceswill not be filled and therefore will allow moisture to flow within theconductor.

Another drawback to this method of applying a moisture blocking materialis that an extrusion head and an extrusion pump for applying thematerial is required for every individual layer of wires used to formthe conductor. The problems described above regarding the regulation ofthe volume of material applied through an extrusion head are multipliedevery time an additional extrusion pump and extrusion head is requiredwithin the conductor manufacturing system. Prior art efforts tomanufacture an acceptable moisture blocked conductor revolved aroundmethods for uniform application of the moisture blocking material to theconductor, but did not solve the problems created by handling andinstallation damage.

Applications of moisture blocking material to the spacing of concentriclay conductors is known within the industry. This can be found in U.S.Pat. Nos. 3,607,487; 3,889,455; 4,105,485; 4,129,466; 4,435,613;4,563,540; and 4,273,597.

U.S. Pat. No. 4,273,597 shows a method of strand filling theinterstitial spacing of a conductor with a powder. This is accomplishedby passing the strands through a fluidized powder bed, where theinterstitial spacing is filled with the powder. The stranded conductorthen exits the opposite end of the bed where an insulating layer isapplied which prevents the powder from vacating the interstitial spacingof the conductor.

U.S. Pat. No. 4,563,540 describes a conductor which is constructed byflooding a waterproofing material among the individual conductors whichmake up the core of the stranded conductor. This flooded core is thenwrapped with a plurality of different layers of shielding material whichprevents the influx of moisture into the stranded conductor.

U.S. Pat. No. 4,435,613 describes a conductor constructed of a pluralityof layers of insulating material with the core (or conducting portion)of the conductor being filled with an insulating layer of polyethylene.This polyethylene layer is contained by other rubber and plastic andepoxy compounds which produce a conductor having a waterproofconstruction.

U.S. Pat. No. 4,129,466 deals with a method for the application of thefilling medium which is applied to a stranded conductor. This methodcomprises a chamber into which are passed individual wires that will beused to form the stranded conductor. These wires have a filling mediumapplied to them in the chamber. After the application of this fillingmedium, the conductor is passed through a chilling chamber where thefilling medium is cooled and allowed to solidify within the interstitialspaces. This method requires that the chamber containing the fillingmedium and the stranded conductor be both heated and pressurized. Theheat applied to the chamber reduces the viscosity of the fillingmaterial, while the pressure assures introduction of the material intothe interstitial spaces of the stranded conductor.

U.S. Pat. No. 4,105,485 deals with the apparatus utilized in the '466method patent previously discussed.

U.S. Pat. No. 3,889,455 discloses a method and apparatus for filling theinterstitial spacing of the stranded conductor in a high temperatureflooding tank. The individual wires are fed into a tank containing thefilling material, the material having been heated to allow it to becomeless viscous. The individual wires are stranded and closed within theconfines of the flooding tank and the finished conductor is withdrawnfrom the opposite end of the flooding tank where it is passed through acooling means. The disadvantages experienced here involve the practiceof stranding the conductor beneath the surface of an elevatedtemperature moisture block pool. No access, either visual or mechanical,to the conductor manufacturing process is practical.

U.S. Pat. No. 3,607,487 describes a method whereby individual strands ofwire are fed into a flooding tank which is supplied with heated fillingmaterial by a pump and an injection means. The stranded conductor iswithdrawn through the opposite end of the flooding tank, wiped in awiping die, wrapped in a core wrapper and then passed through a binderwhere it is bound. The bound, wrapped core is then passed through acooler which sets the filling material. The above described process isrepeated through another flooding tank, another cooler, another bindingmachine, another flooding tank, another extruder, another coolingtrough, and is eventually withdrawn from the end of the manufacturingline as a product having a plurality of layers of moisture blockingcompound which protects the conductor core. The disadvantages herecomprise a complex manufacturing line whereby moisture blocking materialis applied at many different locations, each having to be meticulouslymonitored and controlled in order for a proper conductor construction tobe obtained.

It can be readily seen from the above referenced methods and apparatusesthat moisture blocked conductors are known and it can also be recognizedthat there are major problems concerning the elimination of moisturecontacting the conductor as a result of handling and installation of acable.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to improvements in insulated solid andstranded cables. An electrical cable and a method for manufacturing theelectrical cable are provided in which a plurality of insulatedconductors have an inner protective layer extruded thereabout, andoutwardly extending ribs, or an exterior ribbed or finned surface, whichincludes a plurality of longitudinally extending ribs or fins betweenwhich exist a plurality of voids. An outer insulation layer may beformed in the same operation as the inner layer or ribs or in asubsequent operation. In a two-pass manufacturing process for thepresent cable, the first pass involves extruding the inner finned layeronto the conductor. The inner layer can be polyethylene, pvc, or anothersuitable plastic material. The inner layer can be cross-linked while itis being applied or batch cross-linked after it is applied. The secondpass involves using a hot melt pumping system to apply the sealantlayer. This system advantageously consists of a Nordson model 550 drummelter which delivers sealant to a CH-440 head through which the cablepasses. Other methods of pumping sealant, applying sealant, and sizingthe sealant layer can be used depending on process or productrequirements. The sealant can be applied over a wide range oftemperatures. Good results are obtained by applying the sealant at about250 degrees Fahrenheit. The outer encapsulating layer is then appliedafter the sealant layer, downstream from the sealant head. The outerlayer can be polyethylene, pvc or another suitable plastic material. Theouter layer can be cross-linked while it is being applied or afterwardsin a batch process.

In a single pass manufacturing process, the conductor is fed into a headthat consists of 3 zones. The inner finned layer is applied in the firstzone. In the second zone the sealant layer is applied. The outerencapsulating layer is applied in the third zone. This process requiresclose control of the sealant temperature. The optimal sealantapplication temperature is from about 200 to about 300 degreesFahrenheit.

In one embodiment of the invention, during manufacture of theself-sealing cable, a material which provides the cable with puncture,crack, and void self-sealing properties is included between the ribs orfins and the outer insulation. The voids are at least partly filled bythe material which will flow into any void, puncture, or crack formed inthe insulation, thus preventing migration of moisture. The self-sealingmaterial is applied in the voids between the ribs or fins and the outerinsulation, therefore the self-sealing material does not contact theconductor.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will be apparent from thefollowing detailed description of the preferred embodiments thereof inconjunction with the accompanying drawings in which:

FIG. 1 is a cut-away, perspective view of a cable of the inventionshowing a stranded conductor, the finned inner layer surrounding theconductor, the insulation, and the area between the fins containing thematerial which provides the self-sealing effect;

FIG. 2 is an end view of the embodiment of the cable shown in FIG. 1;

FIG. 3 is a cut-away side view of the cable shown in FIG. 1.

FIG. 4 is a diagrammatic representation showing insulation damage.

FIG. 5 depicts the soil-filled box used to determine current leakage ina damaged cable.

FIG. 6 is a graph of sample leakage current measurements.

FIG. 7 is a graph of conductor resistance measurements.

FIG. 8 is a graph of sample temperature measurements.

FIG. 9 is a comparison of samples of the invention and a control after91 days in the test.

FIG. 10 is a close-up of the control sample after 91 days in the test.

FIG. 11 is a close-up of the test sample of the present invention after91 days in the test.

FIG. 12 is an end view of an embodiment of the cable of the presentinvention.

DETAILED DESCRIPTION IN THE INVENTION

Although the principles of the present invention are applicable todifferent types of electric cables, the invention will be described inconnection with a known cable structure, such as a 600 volt cable, whichnormally comprises, as a minimum:

-   -   (1) A central conductor of stranded wires of a good conductivity        metal such as copper, aluminum, copper alloys or aluminum        alloys; and    -   (2) A layer of insulation around the stranded conductors which        has been extruded thereover.

FIG. 1 shows a cable 11 comprising a conductor 12 of stranded wires ofcopper or aluminum or alloys thereof. An inner layer 14 encircles cable11. A plurality of longitudinally extending fins or ribs 15 are formedbetween which extend a plurality of voids 16. A layer 10 of materialwhich provides the self-sealing effect is applied in and at least partlyfills voids 16 between ribs 15, inner layer 14, and an outer insulationjacket 13. Insulation jacket 13 is of known material and is preferablyan extruded polymeric material.

Material 10 comprises an elastomer material which can be readily pumpedat temperatures at least as low as 60° C. Preferably, the material has alow molecular weight. Advantageously the material comprises rubberresins and other elastomeric resins, optionally with the addition ofhydrocarbon materials which modify the flow characteristics at desiredtemperatures. Butyl rubber, styrene butadiene rubber, ethylene propylenediene monomer rubber, natural rubber, polyolefinic elastomers, andcombinations thereof are preferred. Optionally the above materials maybe modified by the addition of fillers to obtain certain flow or otherphysical property characteristics. The addition of fillers to change thedensity of the sealant material is advantageous. The addition of glassor ceramic microspheres as a filler is also advantageous. Preferablysuch microspheres have a density range of from about 0.1grams/milliliter to about 0.8 grams/milliliter. The elastomer materialmay advantageously include up to about 60% flow modifiers, includingpolyisobutene and/or isobutene. Preferably the elastomer materialincludes from about 25% to about 60% flow modifiers and is flowable atabout −20° C. when optionally so modified. Other materials, orcombinations of materials, with or without such additions and/orfillers, having such characteristics may also be useful in the presentinvention. A material which has been found to be particularly suitableis butyl rubber.

The preferred material of the present invention has very little or nosignificant Shore A hardness. A test of determining whether or not thematerial has acceptable properties is the Penetrometer Test incorporatedin ASTM D5 Penetration of Bituminous Materials. The 100 grams needlepenetration value at 25° C. should be greater than about 100 tenths of amillimeter.

The material used to provide the self-sealing effect to the electriccable of the present invention has the following properties:

-   -   (a) The material is substantially insoluble in water;    -   (b) The material is a dielectric, i.e., it is non-conductive and        is not a semi-conductor;    -   (c) The material causes the cable to be self-sealing, i.e., it        will flow, at ambient temperature, into insulation voids and/or        cracks and prevent contact between the conductor and moisture        which could cause cable failure; and    -   (d) The material does not absorb moisture or swell upon contact        with moisture.

In the preferred embodiment of the present invention, the material usedto at least partly fill voids 16 is a compound having a low molecularweight. Preferably, the material is butyl rubber. Advantageously thereis little or no air present between voids 16 and insulation jacket 13.

The material of the present invention may optionally contain fillermaterial, but is essentially free of any solvents or oils.

The cable 11 described in connection with FIG. 1 can be used withoutfurther layers encircling the insulation jacket 13.

Also, in other embodiments of the present invention described herein,the conductor and layers of insulation can be the same as thosedescribed in connection with FIG. 1.

The cable 11 illustrated in FIG. 2 is an end view of the cableillustrated in FIG. 1.

FIG. 3 is a cut-away side view of cable 11 shown in FIG. 1 andillustrates voids 16 and ribs or fins 15.

The ratio for the height of the fins to the width can vary. The mostdesirable height/width ratio ranges from about 0.25 to about 4.00. Thebase or top thickness at the point where the fins contact the rest ofthe materials can increase or decrease from the average fin thicknessbut should not be less than about 5 percent of the average width of thefins from top to bottom.

The ratio for the height of fins 15 to the width of voids 16 can vary.Advantageously, the height to width ratio ranges from about 0.25 toabout 4.00. Preferably the height to width ratio ranges from about 0.1to about 1.00. The fins do not have to be equally spaced but it isgenerally desirable to equally space the fins to achieve equaldistribution of the medium that is in the channel regions, voids 16, andimprove cable concentricity. The number of fins can range from a minimumof 2 up to any practical number that is needed based on the size of thecable, structural needs of the cable, the material being used in thevoids, the delivery rate needed if applicable for the material, or thephysical size of the material being delivered. The base thickness canvary based on thickness requirements of industry specifications,structural needs of the cable, or other specific cable needs.

The retaining mechanism between the outer encapsulating jacket orinsulation and the fins can be a polymeric bond between the outerextruded layer 13 and the fins 15, or may be purely frictional. Thefrictional mechanism is due to the compressive forces, surface area, andfrictional coefficient between the two layers. A material can be addedduring processing that increases the frictional coefficient between thetwo layers, if a polymeric bond is desired, it should constitute bondingof at least 50% of the exposed surface area of fins 15, i.e., the upperportion of the fins that: contact the interior surface of the outerextended layer 13. Another retaining mechanism is similar to a shaft anda key, i.e., the upper portion of the fin is embedded into the outerencapsulating layer which helps prevent rotation of the inner layer orother movement. Advantageously the fin is embedded to a depth of atleast about 0.001 inch into the interior of the outer insulation layer,preferably from about 0.002 inch to about 0.005 inch. The embedment canbe varied by controlling different variables of the process. It is alsopossible to have combinations of polymeric, frictional, and embedded finretaining mechanisms between the two layers. Fins 15 may be attached toinner layer 14, outer layer 13, or both.

Materials that can be delivered in the channels in addition to sealingmaterials may include fiber optics, heat transfer fluids to enhancecable heat transfer properties, other desirable materials that wouldprovide a beneficial cable property or use the cable as a messenger toconnect a beginning and/or end point.

The most desirable materials for use as the inner layer 14, fins 15, andouter encapsulating layer 13 are plastics that can be either thermosetor thermoplastic. Known plastic materials can be used in order toachieve desired cable properties.

The colors of the inner layer 14, fins 15, and outer layer 13 materialscan be the same or they may differ. Different colors may be used toallow easier identification of the product in the field or for otherdesirable cable properties. The tins or ribs may be straight, mayspiral, may oscillate about the axis of the cable, or may form differentpatterns depending on the desired cable characteristics and efficiencyand flowability of the sealing material used.

It is to be understood that additional embodiments may includeadditional layers of protective material between the conductor and theinsulation jacket, including an additional water barrier of a polymersheet or film, in which ease it is not essential that the jacket tightlyenclose the layers there within or enter into the spaces between thewires and protective materials, i.e. the interior size of the jacket canbe essentially equal to the exterior size of the elongated elements sothat compression of the elongated elements, and hence, indentation ofthe layers there within including the insulation, is prevented.

The cable of the present invention is of particular advantage in thatnot only does the material fill the space between the inner layer andthe insulation as the cable is manufactured, but after the cable isplaced in service the material will flow into any cuts or puncturesformed as a result of damage during handling and installation of thecable or its use in service. The stresses placed on the conductor andthe insulation during handling and installation of the cable, such asbending, stretching, reeling and unreeling, striking with digging andinstallation equipment can form cuts or punctures in the insulation andbetween the insulation and the conductor. Such cuts or punctures canalso be formed after the cable has been placed in service as a result ofdamage from adjacent utilities, homer owners, or lightening strikes.

The cable of the present invention can provide acceptable service evenafter the insulation has been cut or punctured, exposing the conductor.In order to determine the efficiency of using a self-sealing materialdefects were made in the insulation layer of two 600 V cable samples. Onone of the cable samples, a layer of butyl rubber was applied beforeapplication of the outer insulation layer of the cable. The other cablesample did not have the butyl rubber layer. Both cable samples wereplaced inside separate 1 liter glass beakers containing tap water. Eachcable sample was energized at 110V to ground with AC current. The samplewhich did not have the butyl rubber layer exhibited severe corrosionovernight. The sample containing the butyl rubber layer exhibited nocorrosion after being energized and submerged for 4 weeks in tap waterin the glass beaker.

Example 1

This test was designed to evaluate the performance of the presentinvention's self sealing, 600 V underground cable. The test program waspatterned after a previously developed procedure to evaluateself-sealing or self-repairing cable designs.

To conduct the test damaged cables were placed in a specially mixed,moist soil. The cables were then energized with 120 V ac to ground.Measurements made included changes in leakage current to earth and cableconductor resistance. The temperature of each cable near the damagepoint was also monitored.

Four control sample replicates and eight self-sealing sample replicateswere evaluated. All four control samples failed the test relativelyearly in the test program. All eight self-sealing samples performedwell, with no significant increase in conductor resistance and lowleakage current values throughout the 60-day test period.

Conventional and self-sealing 600 volt underground cable with a 2/0 AWGcombination unilay aluminum conductor were tested in 10-foot lengths.

The soil used in the test was a mixture of Ottawa Sand, Wyo. Bentoniteand fertilizer. The combination of the three materials provides asandy-silt type soil, which is very conductive. The sand serves as thebasic soil structure while the silt provides small particles that canwork their way into the damaged areas of the cable. The silt also helpsto keep water evenly dispersed throughout the soil. The fertilizerenhances the conductivity of the soil and may enhance corrosion as well.The goal was to achieve a soil electrical resistivity of <50 ohmmeters.

Tap water was used to achieve a moisture content near saturation. Thiscombination of soil materials provides a worst case condition for the accorrosion of the aluminum conductor in 600 V underground cables and isalso repeatable from lab to lab. The soil mixture was:

100 lbs. Ottawa Sand3.33 lbs. Bentonite23.33 lbs. Tap Water1.26 lbs. of Peters 20-20-20 Plant Fertilizer (mixed with the waterbefore added to the sand and clay ingredients)The amount of water added achieved near saturation conditions. The wetdensity was approximately 127 lbs./ft.

The aging box was made of wood and lined with polyethylene to holdmoisture. The approximate inside dimensions were 6.5 feet long by 1.3feet wide by 1 foot high. A wide, copper tape ground electrode coveredthe bottom and sides of the box on top of the polyethylene. A wireconnected this electrode to ground.

After moist soil was packed in the bottom of the box (approximately 6inches), four control samples and eight self-sealing samples wereinstalled, approximately six inches apart. The two sample sets were:

-   Samples 1-4: conventional 600 V UD wire (control samples) all with    slot damage at the center of the sample-   Samples 5-12: self-sealing cable—all with slot damage near the    center of the sample

Immediately before the samples were placed in the box, they were damageddown to the conductor. One damage condition was used. It consisted of aslot cut into the insulation down to the conductor, perpendicular to thecable axis. A razor knife and an angle guide was used to control theslot size. The size and shape of the damage location is shown in FIG. 4.The damage locations were staggered so they were not adjacent to eachother.

The 10-foot long self-sealing samples were first damaged in the middle.After 5 minutes, they were placed in the box with the damage facing up.They were then covered with soil.

The control samples were initially 2.5-foot long. They were also damagedin the middle, then installed in the box. There was no waiting periodbefore they were covered with soil.

As each sample was installed, a type T thermocouple with a welded beadwas attached to the cable surface, approximately one inch from thedamage location. Once all samples were installed, the soil wascompacted. After 24 hours, the ends were cut of the self-sealing samplesso they were the same length as the control samples. The test layout isshown in FIG. 5.

After the installation was complete, the soil was covered withpolyethylene to minimize the evaporation of water from the soil. 120 Vac was applied continuously to all sample conductors. The soil wasgrounded via the copper ground mat lining the tank. The data collectionwas as follows:

-   1) Measurements (Measured initially, then daily for first 5    workdays; then on Monday, Wednesday and Friday of each week    thereafter.)    -   a) Conductor resistance, each sample individually—Biddle DLRO,        CQ #1010 (Expected accuracy: ±3% of reading)    -   b) Leakage to ground @ 120 V, each sample individually—Fluke 87,        CN 4007 (Expected accuracy: ±3% of reading)    -   c) Sample surface temperature—Yokaggawa C 100, CN 4015 (Expected        accuracy: ±2 Deg. C.)-   2) The test ran for 91 days. When significant degradation occurred    on a sample, it was disconnected from the voltage source.    Significant degradation is defined as:    -   a) Several days with leakage current greater than 1 amp on an        individual sample    -   b) Conductor resistance on an individual sample 10 times greater        than starting resistance-   3) Final soil electrical resistivity and moisture content was    measured when the test was completed.-   4) All measurements were recorded and resistance, leakage and    temperature data were plotted using an Excel spreadsheet.

During the first 26 days of the test the conductor resistance and theleakage current into the soil increased significantly on all fourcontrol samples. They were each removed from the test (disconnected fromthe test voltage) as the conductor resistance exceeded 1,000 micro-ohms.The conductor resistance and the leakage current to the soil for theeight self-sealing samples did not change significantly during the test.

The soil electrical resistivity was measured at the end of the test byplacing a sample of the soil in a 17-inch long, 2-inch inside diameterPVC tube. It was packed to the same density used in the test tank.Two-inch diameter copper plate electrodes were pressed against the soilon each end of the tube. 120 volts ac was applied across the electrodesand the resulting current was measured. The current and voltage wereused to calculate the sample resistance, which was then converted toresistivity.

Moisture content and density were measured at the beginning and end ofthe test. To make the measurement, a soil sample was taken using a 1/30cubic foot metal shelby tube. The sample was then oven dried tocalculate moisture and density. The measured weights were used tocalculate moisture content.

Soil resistivity, moisture and density measurements are summarized inTable 1.

TABLE 1 Time of Electrical Resistivity Moisture Content Wet DensityMeasurement (ohm-meters) (% by weight) (lbs./ft³) Initial 4.3 nearsaturation 126 Final 5.1 15.8 126

The insulation resistance, conductor resistance and sample temperaturemeasurements made during the test are shown in FIGS. 6-8. The samplesare identified as S1, S2, 53, etc. The first four are control, theremaining eight are self-sealing. In addition, C=Control,SS=Self-Sealing.

During periods of relatively high leakage current on the control samplesthe temperature of these samples was also relatively high. Photos of thesamples under test are shown in FIGS. 5, 6 and 7. From the photos it isobvious that the control samples experienced significant corrosion whilethe self-sealing samples experienced no noticeable corrosion.

A cyclic current load was applied to the finned cable of the presentinvention using the sealant of the present invention and compared to afinned cable of the present invention using prior art sealant todetermine if the resulting thermal load and head pressure would causethe sealant to drip from the exposed ends. Fifty-foot cable samples fromour production line were hung vertically in the air supported on bothends with standard electrical connections.

The cables were heated by running the required current through the cableto maintain 50° C. for one week. The same cables were then heated to 75°C. and held for 24 hours. Finally, the cables were heated to 130° C. andheld for a final 24 hours. The temperature measurements were done with athermocouple mounted near the bottom end of each cable.

The prior art cables leaked out and dripped onto the floor the first dayduring the 50° C. test. The cable of the present invention had somemovement of the sealant out of the cable, but no dripping onto the flooroccurred throughout the testing.

Although preferred embodiments of the present invention have beendescribed and illustrated, it will be apparent to those skilled in theart that various modifications may be made without departing from theprinciples of the invention.

1-28. (canceled)
 29. A method of making a self-sealing electrical cable,the method comprising: (a) providing a stranded conductor; (b) forming amulti-layer flow comprising: an inner layer, an outer layer, a pluralityof fins connecting the inner layer and the outer layer, and a sealantmaterial disposed between the inner layer, the outer layer, and theplurality of fins; and (c) applying the multi-layer flow onto thestranded conductor; wherein the multi-layer flow is formed prior tobeing applied onto the stranded conductor.
 30. The method of claim 29,wherein the stranded conductor comprises copper, aluminum, copperalloys, or aluminum alloys.
 31. The method of claim 29, wherein theself-sealing electrical cable is a 600 volt cable.
 32. The method ofclaim 29, wherein the inner layer comprises a thermoplastic or athermoset.
 33. The method of claim 29, wherein the inner layer comprisesa polyethylene or a PVC.
 34. The method of claim 29, wherein the outerlayer comprises a thermoplastic or a thermoset.
 35. The method of claim29, wherein the outer layer comprises a polyethylene or a PVC.
 36. Themethod of claim 29, wherein the sealant material comprises an elastomermaterial.
 37. The method of claim 36, wherein the elastomer material isa dielectric.
 38. The method of claim 36, wherein the elastomer materialdoes not absorb moisture or swell upon contact with moisture.
 39. Themethod of claim 36, wherein the elastomer material has a 100 gram needlepenetration value greater than 100 tenths of a millimeter at 25° C. 40.The method of claim 36, wherein the elastomer material is a butylrubber.
 41. The method of claim 29, wherein the sealant materialcomprises a flow modifier.
 42. The method of claim 41, wherein thesealant material comprises from about 25% to about 60% flow modifiers.43. The method of claim 41, wherein the flow modifier comprises apolyisobutene, isobutene, or a combination thereof.
 44. The method ofclaim 29, wherein the sealant material comprises a filler.
 45. Themethod of claim 44, wherein the filler comprises glass or ceramicmicrospheres.
 46. The method of claim 29, wherein the sealant materialcomprises a polyisobutene.
 47. The method of claim 29, wherein thesealant material comprises a butyl rubber, a styrene butadiene rubber,an ethylene propylene diene monomer rubber, a natural rubber, apolyolefin elastomer, or a combination thereof.
 48. The method of claim29, wherein the sealant material comprises a butyl rubber.
 49. Themethod of claim 29, wherein the sealant material is flowable at about−20° C.
 50. The method of claim 29, wherein the self-sealing electricalcable has, per 50 feet of cable, initially less than about 0.2 inchshrinkback of the inner layer and the outer layer after performing acomplete circular cut of the inner layer and the outer layer.
 51. Themethod of claim 29, wherein the self-sealing electrical cable has, per50 feet of cable, less than about 0.5 inch shrinkback of the inner layerand the outer layer subsequent to accomplishing a complete circular cutof the inner layer and the outer layer and aging for one week.
 52. Themethod of claim 29, wherein the multi-layer flow is formed within amulti-layer extrusion head prior to be applied onto the strandedconductor.
 53. The method of claim 29, wherein the self-sealingelectrical cable comprises at least 2 fins.
 54. The method of claim 29,wherein the self-sealing electrical cable comprises 6 fins.
 55. Themethod of claim 29, wherein the fins comprise a portion of the innerlayer and a portion of the outer layer.
 56. The method of claim 29,wherein the composition of the inner layer and the outer layer aredifferent.
 57. A method of making a self-sealing electrical cable, themethod comprising: (a) providing a conductor comprising copper,aluminum, copper alloys, or aluminum alloys; (b) forming a multi-layerflow comprising: an inner layer comprising a thermoplastic or athermoset, an outer layer comprising a thermoplastic or a thermoset, aplurality of fins connecting the inner layer and the outer layer, and asealant material disposed between the inner layer, the outer layer, andthe plurality of fins; and (c) applying the multi-layer flow onto theconductor; wherein the multi-layer flow is formed prior to being appliedonto the conductor.
 58. The method of claim 57, wherein the sealantmaterial comprises a polyisobutene.
 59. The method of claim 57, whereinthe sealant material comprises a butyl rubber, a styrene butadienerubber, an ethylene propylene diene monomer rubber, a natural rubber, apolyolefin elastomer, or a combination thereof.